Several people told me that the instructions
here do not work with the latest 1.5.0_10 from Sun. Use 1.5.0_09-fcs
for the time being. As you may know Sun is releasing the JDK source
code and for this reason alone the whole idea of "compat" may not
be a long term vision.

Fedora Core 5 Linux distribution comes with the GNU Java.
This is a part of the popular GNU C compiler suite that also
includes support for other programming languages, like Fortran.
The GNU Java now supports most of the features of Sun Java 1.4
and in many cases can be augmented by additional open source
packages to be almost functionally equivalent to the latest JDK from Sun.
It has also some important advantages (e.g., option to produce
native machine code) but it is by necessity behind with the features
of the latest Java Development Kit from Sun. More information can
be found at the GNU Compiler for the Java page at:
http://gcc.gnu.org/java/.
Recently Sun announced that it is legal to redistribute their
Java Development Kit, however, it is unlikely that it will be
a part of Fedora.
The reason for this is that Sun Java Development Kit is
not provided with the source code, while the Fedora Core Linux
only provides packages that are open source.

In most cases you will be fine with the
Open Source, GNU Java. Most open source projects that
need Java Development Kit do work with the GNU Java.
But some do not... Especially those that push latest standards
to the limits and require the latest features of the Java SDK
from Sun. With the race to new standards for Java, you may need
the Sun's Java SDK, but you also do not want to break
the code that assumes that GNU Java is available on your machine.
You will definitely need the Sun JDK if you use their latest
and greatest IDEs or Enterprise type solutions.
But YOU DO NOT WANT TO BRUTALLY DELETE/UNINSTALL the GNU Java
or just put the Sun JDK into /opt and change the
$JAVA_HOME and $CLASSPATH. Luckily there
is a nice solution via alternatives package that
comes with Fedora. We are even more lucky that there is
a JPackage.org project and that Fedora
supports it.

Make Sun JDK your default

Start from becoming a root by logging in
as root or by sudo or simply by:

su -

and get the latest JDK from Sun
http://java.sun.com/downloads.
Select the Java 2 Platform, Standard Edition (J2SE).
Choose the latest edition (at this writing it was 1_5_0_07,
i.e., JDK 5.0 Update 7) and get the Linux RPM in self-extracting
file (DO NOT DOWNLOAD Linux self-extracting file!). You should
get a file like jdk-1_5_0_07-linux-i586-rpm.bin
that is about 45 MBytes in size. If it does not have
the rpm.bin at the end, you took the wrong one.Note: There are many approaches to installing Sun JDK under
Linux. Some go through the route of building the RPM with paths
and directories suitable for Fedora. What I describe here is
a different approach in which the original Sun JDK RPM file
is used but it is brought into compliance with Fedora via
creating symbolic links and the alternatives command
through the JPackage.org RPM package.

While you are downloading this large file,
please downloads the keys for the JPackage repository:

rpm --import http://jpackage.org/jpackage.asc

and install the JPackage repository information
for the yum:

cd /etc/yum.repos.d
wget http://jpackage.org/jpackage.repo

If it did not work, maybe you do not have wget.
You can check it with:

rpm -qi wget

and if you do not have it, install it with:

yum install wget

and try again to get the jpackage.repo file.
The jpackage.repo file comes with the jpackage-general
repository enabled. However, I personally disabled the automatic
retrieval of updates and packages from the
JPackage.org
repository by editing the /etc/yum.repos.d/jpackage.repo
file and changing line(s) enabled=1 to the
enabled=0 . I do not want to
risk the situation when, due to some dependency, the new Java packages
will be installed on my machine without my knowledge during
updates that are run automatically at night. Moreover,
Fedora Core 5 default Java packages are already in the
Fedora repositories (i.e., were officially blessed
by Fedora team), and it is probably better to
try these repositories first than to go directly to the JPackage.org and
risk messing up some naming convention and/or dependencies.
At the same time, JPackage repository can be always enabled
in yum by adding the:--enablerepo=jpackage-generic
--enablerepo=jpackage-generic-nonfree
options on the
command line. Note... There are Fedora repositories on JPackage.org
but there are no files in them from what I see, and there is no
repository for the FC5, so do not enable them.
Since you will be dealing with RPMs, make also sure that you have packages
to manipulate them:

rpm -qi rpm-build
rpm -qi fedora-rpmdevtools

If you do not have them, install them:

yum install fedora-rpmdevtools
yum install rpm-build

Check if you have packages
for GNU Java compiler/libraries installed, since you most likely
need to have some symmetry
between GNU and Sun JDKs so the alternatives
is not confused. But, frankly, I do not know the intrinsics of it,
though it cannot hurt and disk space is cheap.

yum list available '*gcj*'

will list the available but not installed yet
pieces of gcj (GNU Java compiler) and you can install
these packages as:

yum install package1package2...

Please read the man page for
alternatives:

man alternatives

When your Sun JDK Linux RPM self-extracting file
finally arrives you need to execute it, since it is a shell script.
It contains the license agreement and the compressed RPM package with
Sun JDK. It will ask you if you agree to the long license. Say yes,
then it will uncompress the RPM with JDK, and then it will install it.
To run it do:

Unfortunately, the Sun RPM package puts files
in different locations than the ones required by Fedora Core 5.
After running the script you will see a new directory
/usr/java/jdk1.5.0_07 with JDK files. Note that the
actual RPM is left in the directory where you ran the Sun's
jdk-1_5_0_07-linux-i586-rpm.bin script, however,
you do not need to process the RPM, since script already did it.
You may, however, use the RPM package if you want to install
the JDK on another machine or if you erased the JDK at some point
with a the RPM's rpm -e jdk-1.5.0_07-fcs command.
Now, you are ready to install the SUN JDK compatibility RPM
from the JPackage.org

This will create a bunch of links in the
/etc/alternatives and /usr/lib/jvm directories
and others to the /usr/java/jdk1.5.0_07 directory where the
Sun JDK distribution resides.
To check which files were affected do:

rpm -q -l java-1.5.0-sun-compat

Check if the Sun JDK is really a default by doing:

java -version

If you get:

java version "1.4.2"
gij (GNU libgcj) version 4.1.1 20060525 (Red Hat 4.1.1-1)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Hit [Enter] key if you want Sun JDK to
be a default, or enter 1 if you want to change back to GNU Java.
The JPackage compatibility should also make you a link for Java plug-in
for Firefox browser. To make sure it did, do:

The Firefox should look for plug-ins in the
/usr/lib/mozilla/plugins directory.
If your Firefox still does not work with Java,
put the link above also in the directory
/usr/lib/firefox-1.5.0.4/plugins,
or whatever is your particular version of Firefox that you can dig out from
the script that runs it: /usr/bin/firefox or whatever is reported
by:

which firefox

There are different distros of Firefox, so you
may be special but for your own sake, use yum to
update Firefox.
Now, there will be times, when you want to get rid of Sun JDK and
its entries for alternatives. DO NOT TOUCH THESE
LINKS WITH YOUR BARE HANDS. Use yum to uninstall the
JPackage Java compatibility package first and then erase
the Sun JDK with rpm:

yum erase java-1.5.0-sun-compat
rpm -e jdk-1.5.0_07-fcs

Be bold... Do not worry...
You can always reinstall it as described above. Of course, you
can choose to use rpm command directly rather than
yum to work with packages. But yum has a lot
of advantages, since it will maintain the packages and upgrades
and makes a lot of checks to see if things will not break.
But sometimes they break. A popular situation is when you
used rpm to install some package (or the package
was installed with an install (older package is
kept) rather than an update (older package is
removed). In this case, yum gets confused with
dependencies and complains, and it does not want to install
a package. To see if this is a case, list all the installed
rpm packages for some package name with a command:

rpm -qa | grep "java" | sort

(Helpful suggestion from Bob Gustafson, Thanks!)

for example. If you see two versions
of the same rpm, just erase the older (i.e., with
the lower revision number) one. Use the

rpm -e full_package_name_with_version

(but skip the .rpm) and then
try yum update
or yum install again. I am telling you this,
since we will definitely have more javas and compats
coming, and the mess happens. For example (at this time
a hypothetical one), if you got stuck with two compat
packages when updates to the JDK were processed:

If you think that I erred, please let me know
and I will be thankful and will correct... I promise... One more thing
(a disclaimer...): If you believed in anything what I said, and lost time
and money, it is your fault... I am telling you again... I may have made
a mistake, and I do not guarantee that anything will work as described...